How displacement, crew, armament, and speed changed by group

The first, second, and third groups grew progressively in surfaced and submerged displacement while their dimensions changed more subtly. The third group added a non-reloadable aft tube and carried thirteen torpedoes, while complement rose from 36 in the first group to 49. Its lower underwater speed meant that added capability came with a performance cost. Without air-independent propulsion or snorkels, these boats had to surface to refresh their air and recharge their batteries. Deck guns therefore remained useful for commerce operations after torpedoes were expended, while anti-aircraft fits evolved as wartime experience overturned assumptions that submarines could surface only at night or face limited enemy air power.